High-performance half-rate encoding apparatus and method for a TDM system

ABSTRACT

In a transmitter for transmitting communication signals across a radio channel, an improved encoder includes a half-rate encoder receiving a digitized speech signal and generating a compressed bit stream at half-rate, and a signal expander receiving the compressed bit stream and generating an expanded bit stream at full-rate for transmission across a radio channel. An improved receiver, receives the transmitted communication signal which is selected from the group consisting of (a) a conventional full-rate encoded digitized speech signal, and (b) a half-rate encoded digitized speech signal including the expanded bit stream. The improved receiver includes a full-rate equalizer, a half-rate equalizer, and a switch initially routing the received digitized speech signal to the full-rate equalizer, wherein the full-rate equalizer demodulates the received digitized speech signal producing a full-rate demodulated signal and dibits of soft information corresponding to the full-rate demodulated signal. An analyzer analyzes the dibits of soft information and controls the switch to route the received digitized speech signal to one of the full-rate and half-rate equalizers based upon the analysis.

FIELD OF THE INVENTION

[0001] The present invention is directed toward an enhanced half-rateencoding and receiving apparatus and method and, more particularly,toward a half-rate encoding and receiving apparatus and method fortransmitting and receiving half-rate encoded speech at a rate normallyassociated with full-rate encoding.

BACKGROUND OF THE INVENTION

[0002] In typical U.S. digital cellular telephone systems, analog voicesignals are sampled and converted to a digital bit stream. In order tosave bandwidth and therefore provide economic advantage, the digital bitstream is compressed by a source encoder before transmission across aradio channel. Generally, cellular systems provide for either full-rateor half-rate encoding of voice signals.

[0003] Full-rate source encoding in a typical GSM (Global System forMobile Communications) system utilizes an LPC (Linear Prediction Coding)encoder with long-term prediction and regular pulse excitation. Theoutput of the LPC encoder is generally 260 bits every 20 milliseconds,to give an encoding rate of 13 kbits/s. The 13 kbits/s signal output bythe LPC encoder is input to a signal expander, for example, a channelencoder, which adds redundancy to the bit stream and thereby increasesits rate to 22.8 kbits/s. The purpose of the redundancy is to minimizethe consequence of bit errors that are often induced by a noisy radiochannel.

[0004] Half-rate GSM source encoding utilizes a VSELP (Vector SumExcited Linear Prediction) encoder whose output is generally 112 bitsevery 20 milliseconds, to give an encoding rate of 5.6 kbits/s. The 5.6kbits/s signal output by the VSELP encoder is expanded by a channelencoder, which adds redundancy to the bit stream and outputs 228 bitsevery 20 milliseconds, thus increasing the bit rate from 5.6 kbits/s to11.4 kbits/s. Again, the purpose of the added redundancy is to minimizetransmission errors.

[0005] Half-rate encoding effectively doubles the capacity of a cellularsystem. Accordingly, established cellular standards provide half-ratecapability as an economic benefit (twice as many revenue generatingusers). However, there is some penalty to be paid in return, namely,lower bit rate source encoding, e.g., half-rate encoding, goeshand-in-hand with reduced audio fidelity.

[0006] The above comparison between full-rate source encoding andhalf-rate source encoding assumes that the RF (Radio Frequency) channelcarrying the encoded signals does not introduce transmission errorsbeyond the correction capability of the receiver receiving such signals.In general, this is a good assumption because cellular systems aretypically designed to provide a relatively high SNR (Signal-to-NoiseRatio), and therefore a relatively low BER (Bit-Error-Rate). However,under conditions of low SNR, a low-bit-rate encoded signal, whetherhalf-rate or full-rate, can suffer severely degraded audio quality.

[0007] This breakdown in performance is important in practice, ascommercial applications are being developed where a radio system,conforming to an established cellular-radio air-interface standard,fails to provide the relatively high SNR on which thehalf-rate/full-rate trade is premised, or which fails to provideadequate SNR for the effective operation of either full or half-rateencoding. Such commercial applications include, but are not limited to,satellite communication systems with a cellular-standard air-interfacewhich inherently has low link margins, as well as extended-rangecellular systems that provide marine telephone service or telephoneservice capable of penetrating an office building, or the like.Moreover, cellular systems often experience episodes of significantlyreduced SNR caused by channel fading and shadowing, which may result indegraded audio quality.

[0008] The present invention is directed toward overcoming one or moreof the above-mentioned problems.

SUMMARY OF THE INVENTION

[0009] In a transmitter for transmitting communication signals across aradio channel, an improved encoder is provided including a half-rateencoder receiving a digitized speech signal and generating a compressedbit stream at half-rate, and a signal expander receiving the compressedbit stream and generating an expanded bit stream at full-rate fortransmission across a radio channel.

[0010] In one form of the improved encoder, the full-rate isapproximately 2× the half-rate.

[0011] In another form of the improved encoder, the signal expanderincludes a repeater repeating each bit in the compressed bit stream togenerate the expanded bit stream.

[0012] In another form of the improved encoder, the signal expanderincludes a repeater repeating the compressed bit stream to generate theexpanded bit stream.

[0013] In another form of the improved encoder, the compressed bitstream includes bits classified as one of critical, important andunimportant. The signal expander includes a plurality of encodersadditionally encoding the compressed bit stream according to bitclassification to generate the expanded bit stream.

[0014] An improved receiver, according to a first embodiment, isprovided for receiving a digitized speech signal transmitted at afull-rate across a radio channel in a wireless communication system, thedigitized speech signal selected from the group consisting of (a) afull-rate encoded digitized speech signal including a stream of binarybits, and (b) a half-rate encoded digitized speech signal including astream of binary bits expanded for transmission at the full-rate byrepeating each bit in the binary bit stream, the improved receiverincluding a full-rate equalizer, a half-rate equalizer, a switchinitially routing the received digitized speech signal to the full-rateequalizer, wherein the full-rate equalizer demodulates the receiveddigitized speech signal producing a full-rate demodulated signal anddibits of information corresponding to the full-rate demodulated signal,and an analyzer analyzing the dibits of information, the analyzercontrolling the switch to route the received digitized speech signal toone of the full-rate and half-rate equalizers based upon said analysis.

[0015] In one form of the improved receiver, the analyzer includes anXNOR gate receiving the dibits of information, the XNOR gate outputtinga logical one if the bits of the dibit are the same and a logical zeroif the bits of the dibit are different, a counter receiving the outputof the XNOR gate, the counter counting the occurrences of logical onesat the output of the XNOR gate, and a threshold detector connected tothe counter, the threshold detector controlling the switch to route thereceived digitized speech signal to the half-rate equalizer if thenumber of logical ones counted by the counter exceeds a threshold value.

[0016] In another form of the improved receiver, the dibits ofinformation include dibits of soft information, each soft dibitincluding soft values. The analyzer includes a multiplier multiplyingthe soft values of the soft dibits together, the multiplier outputting apositive value if the soft values of the soft dibit are of like polarityand negative value if the soft values of the soft dibit are of differentpolarity, a summer receiving and summing the output of the multiplier,and a threshold detector connected to the summer, the threshold detectorcontrolling the switch to route the received digitized speech signal tothe half-rate equalizer if the summed value exceeds a positive thresholdvalue.

[0017] In another form of the improved receiver, the dibits ofinformation include dibits of soft information plottable on adifferential constellation having real and imaginary axes, each plottedsoft dibit representing a complex value of a differential symbol. Theanalyzer includes a rotator rotating the differential symbols by π/4,the rotated differential symbols having components on the real andimaginary axes, a summer summing magnitudes of the rotated differentialsymbol components on the real and imaginary axes and calculating a ratioof real axis summed magnitudes versus imaginary axis summed magnitudes,and a threshold detector connected to the summer, the threshold detectorcontrolling the switch to route the received digitized speech signal tothe half-rate equalizer if the calculated ratio exceeds a thresholdvalue.

[0018] In another form of the improved receiver, the analyzer includes arotator rotating the differential symbols by π/4, the rotateddifferential symbols having components on the real and imaginary axes, asquarer and summer squaring the complex values of the rotateddifferential symbols and summing the squared values, and a thresholddetector connected to the squarer and summer, the threshold detectorcontrolling the switch to route the received digitized speech signal tothe half-rate equalizer if the real component of the squared/summedvalue exceeds a threshold value.

[0019] In another form of the improved receiver, the dibits of softinformation include a real number magnitude and a real number phaseplottable on a differential constellation, each plotted dibitrepresenting a differential symbol. The analyzer includes a phasedifferentiator determining phase changes between successive differentialsymbols, a summer summing the phase changes determined by the phasedifferentiator, and a threshold detector connected to the summer, thethreshold detector controlling the switch to route the receiveddigitized speech signal to the half-rate equalizer if the summed phasechanges exceed a threshold value.

[0020] An improved receiver, according to a second embodiment, isprovided for receiving a digitized speech signal transmitted at afull-rate across a radio channel in a wireless communication system, thedigitized speech signal selected from the group consisting of (a) afull-rate encoded digitized speech signal including a stream of binarybits, and (b) a half-rate encoded digitized speech signal including astream of binary bits expanded for transmission at the full-rate, theimproved receiver including a full-rate demodulation branch including afull-rate equalizer and a first CRC (Cyclic Redundancy Check) decoder, ahalf-rate demodulation branch including a half-rate equalizer and asecond CRC decoder, a switch receiving the digitized speech signal andinitially routing the received digitized speech signal to both thefull-rate and half-rate demodulation branches, wherein the receiveddigitized speech signal is received by the full-rate and half-rateequalizers producing full-rate demodulated and half-rate demodulatedsignals, respectively. The full-rate demodulated signal input to thefirst CRC decoder performing a CRC check on the full-rate demodulatedsignal and producing a first CRC check signal. The half-rate demodulatedsignal input to the second CRC decoder performing a CRC check on thehalf-rate demodulated signal and producing a second CRC check signal.The analyzer analyzes the first and second CRC check signals, andcontrols the switch to route the received digitized speech signal to oneof the first and second demodulation branches based on the analysis.

[0021] A method of transmitting a digitized signal across a radiochannel is provided including steps of encoding a digitized signal at afirst rate, expanding the encoded digitized signal to a second rategreater than the first rate, and transmitting the expanded digitizedsignal at the second rate across a radio channel.

[0022] In one form of the transmitting method, the encoded digitizedsignal includes a binary bit stream, and the step of expanding theencoded digitized signal to a second rate greater than the first rateincludes the step of repeating each bit in the binary bit stream.

[0023] In another form of the transmitting method, the encoded digitizedsignal includes a binary bit stream, and the step of expanding theencoded digitized signal to a second rate greater than the first rateincludes the step of repeating the binary bit stream.

[0024] In another form of the transmitting method, the encoded digitizedsignal includes a binary bit stream having bits classified as one ofcritical, important and unimportant, and the step of expanding theencoded digitized signal to a second rate greater than the first rateincludes the step of additionally encoding the binary bit streamaccording to bit classification.

[0025] In another form of the transmitting method, the step ofadditionally encoding the binary bit stream according to bitclassification includes the steps of deriving parity bits from thecritical bits, encoding the parity bits to produce a first outputsignal, combining the critical and important bits and adding six tailbits to produce a second output signal, encoding the second outputsignal to produce a third output signal, encoding the unimportant bitsto produce a fourth output signal, and combining the first, third andfourth output signals to produce the expanded digitized signal at thesecond rate.

[0026] In another form of the transmitting method, the step of encodingthe parity bits to produce a first output signal includes the step ofencoding the parity bits with a ⅙ rate convolutional encoder, the stepof encoding the second output signal to produce a third output signalincludes the step of encoding the second output signal with a ¼ rateconvolutional encoder, and the step of encoding the unimportant bits toproduce a fourth output signal includes the step of encoding theunimportant bits with a ½ rate convolutional encoder.

[0027] In another form of the transmitting method, the digitized signalincludes a TDM (Time Division Multiplex) signal.

[0028] In another form of the transmitting method, the second rate isapproximately 2× the first rate.

[0029] A method of receiving a digitized speech signal transmitted at afull-rate across a radio channel in a wireless communication system isprovided, the digitized speech signal selected from the group consistingof (a) a full-rate encoded digitized speech signal including a stream ofbinary bits, and (b) a half-rate encoded digitized speech signalincluding a stream of binary bits expanded for transmission at thefull-rate by repeating each bit in the binary bit stream, the methodincluding the steps of receiving the digitized speech signal at areceiver, determining whether the received digitized speech signal isthe full-rate or half-rate encoded digitized speech signal, andactivating either a full-rate or a half-rate equalizer at the receiverin response to the determination to demodulate the received digitizedspeech signal.

[0030] In one form of the receiving method, the steps of determiningwhether the received digitized speech signal is the full-rate orhalf-rate encoded digitized speech signal and activating either afull-rate equalizer or a half-rate equalizer at the receiver in responseto the determination to demodulate the received digitized speech signalinclude the steps of demodulating the received digitized speech signalat the full-rate equalizer, the full-rate equalizer producing dibits ofinformation in response thereto, inputting the dibits of information toan XNOR gate, the XNOR gate outputting a logical one if the bits of thedibit are the same and a logical zero if the bits of the dibit aredifferent, counting the number of occurrences of logical ones at theoutput of the XNOR gate, and activating the half-rate equalizer todemodulate the received digitized speech signal if the number of logicalones exceeds a threshold value.

[0031] In another form of the receiving method, the steps of determiningwhether the received digitized speech signal is the full-rate orhalf-rate encoded digitized speech signal and activating either afull-rate equalizer or a half-rate equalizer at the receiver in responseto the determination to demodulate the received digitized speech signalinclude the steps of demodulating the received digitized speech signalat the full-rate equalizer, the full-rate equalizer producing dibits ofsoft information in response thereto, each soft dibit including softvalues, multiplying the soft values of the soft dibit together at amultiplier, the multiplier outputting a positive value if the softvalues of the soft dibit are the same and a negative value if the softvalues of the soft dibit are different, summing the output of themultiplier, and activating the half-rate equalizer to demodulate thereceived digitized speech signal if the summed output of the multiplierexceeds a threshold value.

[0032] In another form of the receiving method, the steps of determiningwhether the received digitized speech signal is the full-rate orhalf-rate encoded digitized speech signal and activating either afull-rate equalizer or a half-rate equalizer at the receiver in responseto the determination to demodulate the received digitized speech signalinclude the steps of demodulating the received digitized speech signalat the full-rate equalizer, the full-rate equalizer producing dibits ofsoft information in response thereto, the soft dibits plottable on adifferential constellation having real and imaginary axes, each plottedsoft dibit representing a complex value of a differential symbol,rotating the differential symbols by π/4, the rotated differentialsymbols having components on the real and imaginary axes, summingmagnitudes of the rotated differential symbol components on the real andimaginary axes, calculating a ratio of real axis summed magnitudesversus imaginary axis summed magnitudes, and activating the half-rateequalizer to demodulate the received digitized speech signal if theratio exceeds a threshold value.

[0033] In another form of the receiving method, the steps of determiningwhether the received digitized speech signal is the full-rate orhalf-rate encoded digitized speech signal and activating either afull-rate equalizer or a half-rate equalizer at the receiver in responseto the determination to demodulate the received digitized speech signalinclude the steps of demodulating the received digitized speech signalat the full-rate equalizer, the full-rate equalizer producing dibits ofsoft information in response thereto, the soft dibits plottable on adifferential constellation having real and imaginary axes, each plottedsoft dibit representing a complex value of a differential symbol,rotating the differential symbols by π/4, the rotated differentialsymbols having components on the real and imaginary axes, squaring thecomplex values of the rotated differential symbols, summing the squaredvalues, and activating the half-rate equalizer to demodulate thereceived digitized speech signal if the real component of thesquared/summed value exceeds a threshold value.

[0034] In another form of the receiving method, the steps of determiningwhether the received digitized speech signal is the full-rate orhalf-rate encoded digitized speech signal and activating either afull-rate equalizer or a half-rate equalizer at the receiver in responseto the determination to demodulate the received digitized speech signalinclude the steps of demodulating the received digitized speech signalat the full-rate equalizer, the full-rate equalizer producing dibits ofsoft information in response thereto, each soft dibit representing adifferential symbol, determining phase differences between successivedifferential symbols, summing the determined phase differences, andactivating the half-rate equalizer to demodulate the received digitizedspeech signal if the summed phase difference exceeds a threshold value.

[0035] In another form of the receiving method, the steps of determiningwhether the received digitized speech signal is the full-rate orhalf-rate encoded digitized speech signal and activating either afull-rate equalizer or a half-rate equalizer at the receiver in responseto the determination to demodulate the received digitized speech signalinclude the steps of demodulating the received digitized speech signalin parallel using both full-rate and half-rate demodulation branches,performing CRC (Cyclic Redundancy Check) checks on the demodulatedfull-rate and half-rate signals, and deactivating one of the full-rateand half-rate demodulation branches in response to the CRC checks.

[0036] A method of establishing voice communication across a radiochannel in a wireless communication system is provided, the methodincluding the steps of transmitting a digitized speech signal at afull-rate across a radio channel, the digitized speech signal selectedfrom the group consisting of (a) a full-rate encoded digitized speechsignal including a stream of binary bits, and (b) a half-rate encodeddigitized speech signal including a stream of binary bits expanded fortransmission at the full-rate by repeating each bit in the binary bitstream, receiving the transmitted digitized speech signal at a receiver,determining whether the received digitized speech signal is thefull-rate or half-rate encoded digitized speech signal, and activatingeither a full-rate equalizer or a half-rate equalizer at the receiver inresponse to the determination to demodulate the received digitizedspeech signal.

[0037] It is an object of the present invention to improve wirelesscommunication performance in low SNR conditions.

[0038] It is a further object of the invention to provide increasedend-to-end performance across radiotelephone links that exhibit sub-parSNR, without requiring extensive and costly changes to the system'sbasic structure.

[0039] It is a further object of the invention to expand the speechoutput of a half-rate source encoder to approximately twice the bit ratenormally associated with half-rate transmission, so that enhancedhalf-rate speech is transmitted at the rate normally associated withfull-rate transmission.

[0040] Other aspects, objects and advantages of the invention can beobtained from a study of the application, the drawings, and the appendedclaims.

BRIEF DESCRIPTION OF THE DRAWINGS

[0041]FIG. 1 illustrates a prior art GSM time-division multi-framepattern;

[0042]FIG. 2 depicts four frames of multi-frame N shown in FIG. 1;

[0043]FIG. 3 illustrates four frames of full-rate encoded transmissionin a prior art GSM TDM (Time Division Multiplexed) system;

[0044]FIG. 4 illustrates four frames of half-rate encoded transmissionin a prior art GSM TDM system;

[0045]FIG. 5 is a curve of speech quality of a transmitted digitizedvoice signal as a function of the source encoding rate;

[0046]FIG. 6 is a curve of speech quality of a transmitted digitizedvoice signal as a function of SNR conditions;

[0047]FIG. 7 is a block diagram of a basic form of an enhanced half-rateencoder;

[0048]FIG. 8 is a curve of speech quality of a transmitted digitizedvoice signal as a function of SNR conditions for both enhanced half-rateencoding according to the present invention and prior art half-rateencoding;

[0049]FIG. 9 is a block diagram of a first embodiment of a communicationsystem with enhanced half-rate encoding;

[0050]FIG. 10 is a plot of the four possible two-bit states on the real(I) and imaginary (Q) axes of a full-rate encoded signal modulated withπ/4-DQPSK (Differential Quadrature Phase Shift Keying) modulation;

[0051]FIG. 11 is the plot of FIG. 10 limited to two possible two-bitstates for a half-rate encoded repeated signal;

[0052]FIG. 12 illustrates soft value computation in the binary equalizershown in FIG. 9;

[0053]FIG. 13 is a block diagram of the de-rotator and equalizer shownin FIG. 9;

[0054]FIG. 14 is a block diagram of one form of the analyzer shown inFIG. 13;

[0055]FIG. 15 is a block diagram of an alternative form of the analyzershown in FIG. 13;

[0056]FIG. 16 is a block diagram of another alternative form of theanalyzer shown in FIG. 13;

[0057]FIG. 17 is a plot of the differential constellation of FIG. 10,rotated by FIG. 18 is a plot of the differential “sub-constellation” ofFIG. 11, rotated by π/4;

[0058]FIG. 19 is a block diagram of still another alternative form ofthe analyzer shown in FIG. 13;

[0059]FIG. 20 is a block diagram of yet another alternative form of theanalyzer shown in FIG. 13;

[0060]FIG. 21 is a block diagram of another embodiment of the receivershown in FIG. 9;

[0061]FIG. 22 is a block diagram of a prior art encoder utilizinghalf-rate encoding;

[0062]FIG. 23 is a block diagram of a modified enhanced half-rateencoder;

[0063]FIG. 24 is a block diagram of a prior art full-rate encoder;

[0064]FIG. 25 is a block diagram of a variant of the enhanced half-rateencoder shown in FIG. 23;

[0065]FIG. 26 is a plot of FER (Frame Error Rate) of transmitteddigitized voice signals as a function of SNR conditions for bothenhanced half-rate encoding (FIG. 25) and prior art full-rate encoding(FIG. 24);

[0066]FIG. 27 is a diagram illustrating operation of a prior artfull-rate encoder;

[0067]FIG. 28 is a diagram illustrating operation of another embodimentof the enhanced half-rate encoder;

[0068]FIG. 29 is a curve of the FER of a transmitted digitized voicesignal as a function of SNR conditions for both enhanced half-rateencoding (FIG. 28) and prior art full-rate encoding (FIG. 27);

[0069]FIG. 30 is a block diagram of a radiotelephone for use with theenhanced half-rate modulation scheme of the present invention; and

[0070]FIG. 31 is a block diagram illustrating the toggling of near endand far end radiotelephones of a conversation pair between prior artencoding (full-rate or half-rate) and enhanced half-rate encoding.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0071]FIG. 1 illustrates how the GSM time-division framing pattern in aTDM system accommodates the bit stream of a source, or channel, encoder.The RF channel is divided into multiple frames, or multi-frames, eachhaving a duration of 120 milliseconds. Each of the multi-frames issubdivided into twenty six TDMA (Time Division Multiple Access) frames,each having a duration 4.615 milliseconds. Twenty four of the TDMAframes carry voice traffic and two, labelled as “C” in FIG. 1, carrycontrol information. Each of the TDMA frames is further divided intoeight time slots of 0.577 milliseconds duration.

[0072] Time slots are the basic construction unit of the radio channelfor end-to-end communication across the radio link. FIG. 2 depicts theeight time slots of each TDMA frame of multi-frame N shown in FIG. 1.Each time slot provides one channel for voice or data communication.

[0073] As shown in FIG. 3, full-rate encoding is provided across achannel by successive occurrences of a given time slot. For instance,the digitized, full-rate encoded and expanded voice communication of aconversation between end users A and B may be transmitted duringsuccessive occurrences of the third time slot in successive frames. Withfull-rate encoding, one carrier accommodates eight channels, each ofwhich supports a full-rate voice encoder.

[0074] On the other hand, as shown in FIG. 4, half-rate encoding isprovided across a channel by occurrences of a given time slot inalternating TDMA frames. For instance, a conversation between end usersC and D may appear on occurrences of the third time slot in odd numberedTDMA frames (1, 3, etc.), whereas a conversation between end users E andF may appear on occurrences of the third time slot in even numbered TDMAframes (2, 4, etc.). With half-rate encoding, an RF carrier providessixteen channels rather than eight. This effectively doubles thecapacity of the system.

[0075] However, as shown in FIG. 5, the penalty to be paid in return fordoubling system capacity is that lower bit rate source encoding, e.g.,half-rate encoding, goes hand-in-hand with reduced audio fidelity.However, as lower bit rate encoding techniques are improved, the penaltyis reduced. This suggests a general principle underlying half-ratesystems, namely, that a good half-rate encoder can capture a large partof the fidelity of a full-rate encoder, but require only half thechannel bandwidth. Thus, the trade off between fidelity and bandwidthbecomes favorable.

[0076] Under conditions of low SNR, as shown in FIG. 6, the performanceof a low-bit-rate encoder, e.g., a half-rate encoder, can break down.This breakdown in performance becomes a concern in systems which fail toprovide adequate SNR for the effective operation of either encodingmethod, such as, but not limited to, satellite communication systemswith a cellular-standard air-interface and extended-range cellularsystems. Moreover, communication systems may experience channel fadingand/or shadowing of the transmitted signal which may cause a significantreduction in SNR, thereby leading to severely degraded audio quality.

[0077] In its basic form, as shown in FIG. 7, the enhanced encoder 99includes a half-rate encoder 100 and a half-to-full rate signal expander102. The half-rate encoder 100 and signal expander 102 are a part of atransmitter 104 of a wireless voice communication apparatus such assatellites, base stations, cellular phones, and the like.

[0078] Digitized speech 106 is encoded according to the half-rate sourceencoder 100. The output 108 of the half-rate encoder 100, at 5.6kbits/s, is expanded by the signal expander 102 to about twice the bitrate normally associated with half-rate transmission, e.g., to 22.8kbits/s rather than 11.4 kbits/s, so that the enhanced half-rate speechsignal 110 output by the signal expander 102 is transmitted at the ratenormally associated with full-rate transmission. As shown in FIG. 8,this enhanced half-rate encoding by enhanced encoder 99 representsapproximately a 3 dB improvement in speech quality of the demodulatedsignal over prior art half-rate encoding.

[0079] A first embodiment of the enhanced encoder, designed for aradiotelephone system that uses DQPSK (Differential Quadrature PhaseShift Keying) modulation, and more specifically for a D-AMPS (DigitalAdvanced Mobile Phone System) cellular telephone system, is shown inFIG. 9. The communication system 120 includes a transmitter 122 and areceiver 124, which may be included in any wireless communicationdevices communicating with each other, such as, but not limited to,satellites, base stations, cellular phones, and the like. Thetransmitter 122 includes a digitized voice signal input at 123 to afull-rate encoder 126 and a half-rate encoder 128, only one of which isactivated at a given time, a CRC (Cyclic Redundancy Check) encoder 130,a ½ rate convolutional encoder 132, an interleaver 134, a repeater 136which is utilized as the signal expander 102 (see FIG. 7) duringenhanced half-rate transmission only, and a modulator 138. Alternately,a switch (not shown) may direct the digitized voice signal 123 to eitherthe full-rate 126 or half-rate 128 encoder.

[0080] If the full-rate encoder 126 is activated, the transmitter 122will transmit via a standard VSELP full-rate scheme. The ACELP(Algebraic Code Excited Linear Prediction) scheme is similar. Thefull-rate encoder 126 produces 77 Class I bits and 82 Class II bitsevery 20 milliseconds (for a rate of 7.95 kbits/s). Twelve of the ClassI bits are encoded using a CRC code by the CRC encoder 130, whichproduces 7 parity bits. The 77 Class I bits, the 7 CRC bits, and 5 tailbits are fed to the ½ rate convolutional encoder 132, which produces 178coded bits. Those and the uncoded Class II bits add up to 260 bitsoutput by the VZ rate convolutional encoder 132 (for a rate of 13kbits/s). The 260 bits are fed to an interleaver 134 which interleavesthe bits. The interleaved bits follow the dotted path 140, bypassing therepeater 136, to the modulator 138 and are modulated into 130 π/4-DQPSKsymbols (each symbol including two bits) in the slot transmission. The260 bits, taken in successive pairs, or “dibits”, are mapped to specificpoints (00,01,10,11) on the differential constellation as shown in FIG.10, which dictates the phase change between symbols in the modulation.

[0081] On the other hand, if the half-rate encoder 128 is activated, 33Class I bits and 40 Class II bits will be produced every 20 millisecondsby the half-rate encoder 128 (for a rate of 3.65 kbits/s). Twelve of theClass I bits are encoded using a CRC code by the CRC encoder 130, whichproduces 7 parity bits. The 33 Class I bits, the 7 CRC bits, and the 5tail bits are fed to the ½ rate convolutional encoder 132, whichproduces 90 coded bits. Those and the uncoded Class II bits add up to130 bits (for a rate of 6.5 kbits/s), exactly half of the 260 bits whichare transmitted with full-rate encoding. The 130 bits are fed to theinterleaver 134 which interleaves the bits. The 130 interleaved bits arethen repeated by the repeater 136, bit by bit, with each data bitproducing two of the same data bits. That is, 1→11 and 0→00. Taken inpairs, i.e., dibits, the resulting 260 bits are modulated by themodulator 138 using π/4-DQPSK modulation. However, the doubling of theoriginal bits after interleaving, guarantees that only two points (00and 11) of the differential constellation are used. The use of this“sub-constellation”, as shown in FIG. 11, gives us a defacto “π/4-DBPSK”(Differential Binary Phase Shift Keying), and gains approximately 3 dBin performance (audio fidelity) due to the increased Euclidean distancebetween only two constellation points as opposed to the fourconstellation points shown in FIG. 10.

[0082] In both the full-rate and enhanced half-rate modes of operationdescribed above, the exact number of bits and their classification,along with the rate of the convolutional encoder 132 are exemplary only.Further, conventional half-rate encoding may be added as an additionalmode of operation by utilizing line 140 to bypass the repeater 136 withthe half-rate encoder 128 activated. Still further, thesub-constellation shown in FIG. 11 is restricted to use over the datafield of the frame structure; all other fields (synchronization, SACCH(Slow Associated Control Channel), CDVCC (Coded Digital VerificationColor Code), etc.) are transmitted utilizing standard full-rateπ/4-DQPSK modulation.

[0083] The enhanced half-rate, or “binary modulation” mode of operationdescribed above, effectively expands the half-rate signal to fill thefull-rate frame structure, and consequently provides approximately a 3dB advantage in audio fidelity.

[0084] Referring to FIG. 9, the transmitted signal 142 is received atthe receiver 124 and the steps performed at the transmitter 122 areessentially performed in reverse. The signal is first de-rotated byde-rotator 143, then demodulated by the equalizer 144, de-interleaved byde-interleaver 146, decoded by convolutional decoder 148, and furtherdecoded with a CRC decoder 150. The resulting received digital signal152 may be further processed for providing the signal in analog form toa user.

[0085] With enhanced half-rate encoding at the transmitter 122, in orderfor the equalizer 144, preferably a Viterbi equalizer, to handle thebinary modulation properly, half of the branches on its trellis need tobe disabled. The remaining parts of the equalizer 144, including thechannel tracker (not shown), require no modification. On the other hand,if a differential detector is utilized at the receiver 124 in place ofthe equalizer 144, no modification is required in order to demodulatethe transmitted binary modulated signal. For convenience, it is assumedthat enhanced half-rate encoding was utilized at the transmitter 122,such that the transmitted signal 142 is a binary signal and theequalizer 144 is a binary equalizer.

[0086] The binary equalizer 144 is preferably a 2-tap channel modelequalizer, typically resulting in a fully connected 4-state trellis.Since the D-AMPS modulation scheme is typically π/4-DQPSK, thede-rotator 154 de-rotates the received binary signal 142 before feedingit to the equalizer 144. De-rotation removes the π/4 shift from theπ/4-DQPSK modulation, effectively resulting in DQPSK modulation.

[0087] In DQPSK, two information bits determine the phase transitionφ_(n) from the present coherent phase θ_(n−1) to the next coherent phaseθ_(n), using the equation

θ_(n)=θ_(n−1)+φ_(n).

[0088] Both the phase transition and the coherent phase belong to theset {0, π/2, π, 3π/2}. The coherent symbols (or constellation points)S_(n), are given by

S _(n) =e ^(jθ) ^(_(n))

[0089] and belong to the set {+1,+j, −1, −j}. One stage of the trellisis described below in Table 1, including the present state and thecoherent symbol and phase associated with the present state and theinformation bits and the phase transition associated with theinformation bits. Each transition ending in a given state is labelledwith the coherent symbol associated with that state, e.g., all fourtransitions ending in state 0 are labelled +1. TABLE 1 Standard trellisfor D-AMPS after de-rotation Information bits (phase transition) Coh.Symbol 00 (0) 01 (π/2) 11 (π) 10 (3π/2) Present State (Phase) Next state0 +1 (0) 0 1 2 3 1 +j (π/2) 1 2 3 0 2 −1 (π) 2 3 0 1 3 −j (3π/2) 3 0 1 2

[0090] With binary modulation, the information bit pairs are restrictedto 00 and 11. In a trellis constrained accordingly, states 0 and 2become disconnected from states 1 and 3. That is, a path on theconstrained trellis that visits state 0 can also visit state 2, but notstates 1 or 3, and vice versa. It is assumed that the subtrellis withstates 0 and 2, i.e., the binary trellis, is the trellis used by thebinary equalizer 144 for the binary demodulation. This is shown in Table2. For the standard trellis, once a path has been chosen, thecorresponding bit pairs are produced by the equalizer as hardinformation. For th binary trellis, the bit pairs 00 and 11 map into theunrepeated bits 0 and 1, respectively, which are produced by the binaryequalizer 144 as hard information. TABLE 2 Modified trellis for binarymodulation after de-rotation Information bits (phase transition) Coh.Symbol 00 (0) 11 (π) Present State (Phase) Next state 0 +1 (0) 0 2 2 −1(π) 2 0

[0091] In addition to hard information, the binary equalizer 144produces soft information, in the form of dibits, which is used by theconvolutional decoder 148 in decoding the received signal. This softinformation is a real number whose value is indicative of the likelihoodof the accuracy of the produced hard information or bit. The softinformation is somewhat complicated because of the differential encodinginherent in π/4-DQPSK modulation. Below is a summary of the operation ofthe binary equalizer 144 in producing soft information.

[0092] In Table 2, the current information bit pair at time n depends onthe current and next coherent symbols. It is assumed that hard decisionshave already been made on the current transition and the next transitionand a hard bit has been output. This is illustrated by the example inFIG. 12, where the surviving path corresponding to the hard decision is

P=(0→0→0),

[0093] which translates to bit 0, i.e., the binary equalizer has outputbit 0 as hard information at time n. In order to obtain a soft value forthe first bit, or MSB (Most Significant Bit) relating to bit 0 at time n(represented by the underlined pair 00), occurrences of bit 1(represented by pair 11) at time n are sought. More specifically, thepath metrics of bit 0 at time n, namely, M_(n) at time n and M_(n+1) attime n+1 (time n+1 being used due to the dependency of the currentinformation bit pair on the next coherent symbol), are compared with themetrics of paths that would have produced bit 1 at time n. Since thebinary trellis of Table 2 is utilized in the binary equalizer 144, thereare only two alternate paths, namely,

P′=(2→0),

[0094] at time n, with path metric M′_(n) (>M_(n), since P is thesurviving path at time n), and

P″=(0→2→0),

[0095] at time n+1, with path metric M″_(n+1) (>M_(n+1), since P is thesurviving path at time n+1). Note that the second transition from 2→0was necessary in path P″ since reliable comparison of metrics requiresthat they have paths ending in the same state (state 0 in the aboveexample). It then follows that a reasonable soft value for the MSB is

min (M′_(n)−M_(n), M″_(n+1)−M_(n+)1).

[0096] Either the full-rate encoder 126 or the half-rate encoder 128 maybe implemented by the transmitter 122 for transmission. Thus, thereceiver 124 needs to be able to tell which transmission mode is beingutilized in order to activate an appropriate equalizer (standard 4-statefor full-rate encoding and modified 2-state for enhanced half-rateencoding) to properly demodulate the received signal. If the receiver124 is aware of which encoding scheme is being utilized by thetransmitter 122, the appropriate equalizer can be easily activated.However, where the receiver 124 is unaware of which encoding scheme isbeing utilized, it must be able to determine which encoding scheme isbeing utilized and activate the appropriate equalizer.

[0097] Referring to FIG. 13, the receiver 124 is illustrated whichdetermines the encoding scheme of the transmitted digital signal 142 andchooses an appropriate equalizer. Upon receipt of the transmitteddigital signal 142, it is de-rotated by de-rotator 154 and fed through aswitch 156 to a standard full-rate equalizer 158 having a 4-statetrellis. The full-rate equalizer 158 is chosen as the starting equalizersince it will operate on the binary modulated signal, albeit with a lossin performance. If binary modulation is used, it is detected by theanalyzer 159 quickly and the half-rate equalizer 160 is activated beforesuffering a significant degradation of the received signal.

[0098] It is assumed that synchronization and downsampling to symbolrate samples have been performed, and the equalizer 158 has demodulatedthe signal producing “soft bits” at the output. These soft bits are realnumbers, i.e., a stream of 0's and 1's, analogous to the projection of adifferential constellation point (see FIG. 10) on the I (real) and Q(imaginary) axes.

[0099] Once demodulation has been performed yielding “soft bits” 162,the soft bits 162 are fed to the analyzer 159 which determines whetherfull-rate encoding or enhanced half-rate encoding has been performed atthe transmitter 122. The analyzer 159 instructs the switch 156 whetherfull-rate or enhanced half-rate encoding has been utilized, and ifenhanced half-rate encoding has been utilized, the analyzer 159 willinstruct the switch 156 to switch to the enhanced half-rate equalizer160 having a 2-state trellis.

[0100] In one form, as shown in FIG. 14, the analyzer 159 performs asimple hard detection of the soft bits, or dibits, i.e., two bits, 162(a negative soft value maps to a logical “1”, and a positive soft valuemaps to a logical “0”). The resulting stream of 0's and 1's can beexamined for the occurrence of double ones and double zeros. An exampleof a circuit which will perform this is an XNOR gate 166 operating oneach bit of a dibit. If the bits of the dibit are the same, the outputof the XNOR gate 166 will be a “1”. If the bits of the dibit aredifferent, the output of the XNOR gate 166 will be a “0”. This outputcan be used to increment a counter 168, which counts the occurrence ofdouble bits. A threshold detector 169 is connected to the counter 168.If the count exceeds the threshold, the threshold detector 169 sends asignal to the switch 156 indicating that enhanced half-rate encoding isbeing used and to switch to the half-rate equalizer 160. This circuitmay be easily implemented in ASIC (Application Specific IntegratedCircuit) circuitry.

[0101] In another form, referring to FIG. 15, the analyzer 159determines the modulation scheme being utilized by multiplying the softvalues of the soft bits 162 of the dibit together. The soft bits 162 ofthe dibit are fed to a multiplier 170 which multiplies the soft values.If the enhanced half-rate modulation scheme is being used, then theadjacent soft values of the dibit will be of like polarity (indicating adouble bit), and the product of the two will be positive. If thefull-rate modulation scheme is being utilized, half of the dibits willhave adjacent soft values of different polarity, and thus these productswill be negative. The output of the multiplier 170 is input to a summer172 which sums the products over all or a portion of the data field ofthe received burst signal. If the full-rate modulation scheme is beingused, the sum will be biased toward zero. Alternatively, if thehalf-rate modulation scheme is being used, the sum will be positivelybiased. Accordingly, the output of the summer 172 is fed to a thresholddetector 174 which employs a threshold to determine the modulationscheme being utilized, and transmits the appropriate signal to theswitch 156.

[0102] In yet another form, referring to FIG. 16, the analyzer 159 takesadvantage of the fact that the differential operation performed by theequalizer 158 on the received coherent symbols produces a differentialconstellation of differential symbols, where the phase of aconstellation point corresponds to the π/4-DQPSK phase change. The softbits (dibits) 162 produced by the equalizer 158 are plotted on adifferential constellation having real and imaginary axes, each plottedsoft dibit represents a complex value of a differential symbol. Thedifferential symbols are rotated by rotator 176 so that theconstellation points of an enhanced half-rate modulated signal lie onthe real (I) axis. Essentially, the rotator 176 multiplies thedifferential symbols of the constellation by a constant, exp jπ/4). FIG.17 illustrates rotation of the differential constellation of FIG. 10 byπ/4, while FIG. 18 illustrates rotation of the differential“sub-constellation” of FIG. 11 by π/4.

[0103] Referring back to FIG. 16, once this rotation is performed,activity on the I and Q axes is observed to determine the modulationscheme being used. If full-rate encoding modulation is being used, therewill be equal energy in the I and Q axes on average, with all symbols ofthe DQPSK constellation being employed. If enhanced half-rate encodingis being used, then almost all of the energy over the data field of thereceived burst signal will be in the I (real) component. This can bequantified by summing the magnitudes of the I and Q components, viasummation block 178, and comparing then in the ratio,$R = {\frac{\sum_{n}{I_{n}}^{2}}{{{{\sum_{n}}Q_{n}}}^{2}}.}$

[0104] If full-rate encoding is being employed, then the ratio R will beclose to 1. If enhanced half-rate encoding is being employed, then theratio R will a number much larger than 1. The output of the summationblock 178, which is the ratio R, is fed to a threshold detector 180which employs a threshold to determine which scheme is being employedand sends the appropriate signal to the switching device 156.

[0105] In still another form, referring to FIG. 19, the analyzer 159again takes advantage of the fact that the differential operationperformed by the equalizer 158 on the received coherent symbols producesa differential constellation of differential symbols, where the phase ofa constellation point corresponds to the π/4-DQPSK phase change. Thesoft bits (dibits) 162 produced by the equalizer 158 are plotted androtated by rotator 181 in the same manner as rotator 176 previouslydescribed with respect to FIG. 16. Once the rotation is performed,activity on the I and Q axes is observed to determine the modulationscheme being used. If full-rate encoding modulation is being used, theaverage value of the squared values of the QPSK samples will be:

avg _(QPSK)=¼(1)²+¼(j) ²+¼(−1)²+¼(−j)²=1+j0.

[0106] If enhanced half-rate encoding is being used, the average valueof the squared values of the BPSK samples will be:

avg _(BPSK)=½(1)²+½(−1)²=1+j0.

[0107] This can be quantified by squaring and summing the complex valuesof the rotated differential symbols, via square and sum block 182.

[0108] If full-rate encoding is being employed, then the real part ofthe squared/summed value will be close to 0. If enhanced half-rateencoding is being employed, then the real part of the squared/summedvalue will be close to 1. The output of the square and sum block 182,which is the real part of the average value, is fed to a thresholddetector 183 which employs a threshold to determine which scheme isbeing employed and sends the appropriate signal to the switching device156.

[0109] In some situations, the complex received data available, i.e.,soft bits 162, is in magnitude-phase format, i.e., in terms of a realnumber magnitude and a real number phase describing the demodulatedconstellation point. Differential detection can be performed without apolar-to-rectangular conversion simply by taking the difference betweenthe phases of successive coherent or differential symbols represented bythe soft dibits. As shown in FIG. 20, in still another form the analyzer159 includes a phase differentiator 184 which determines the differencebetween the phases of successive differential symbols. The phasedifference information from the phase differentiator 184 is analyzed todetermine the type of encoding being used. If full-rate encoding isbeing used, then the average phase change will beΔφ_(avg)=¼(π/4)+¼(−π/4)+¼(3π/4)+¼(−3π/4)=0, since all differentialsymbols (phase changes) occur with equal likelihood. If enhancedhalf-rate encoding is being used, then the average phase change will beΔφ_(avg)=½(π/4)+½ (−3π/4)=π/4. The output of the phase differentiator isfed to a summation block 185 which estimates the average phase changeover the data field by summing the phase changes at the output of thedifferentiator 184. If full-rate encoding is being employed, then thesum of the phase changes will be close to 0. Otherwise, if enhancedhalf-rate encoding is being used, the sum will be close to π/4. Theoutput of the summation block 185 is fed to a threshold detector 186which utilizes a threshold value to determine whether enhanced half-rateencoding is being employed and transmits an appropriate signal to theswitching device 156. Attention should be paid, however, to the modulonature of the angle measurement in making this accumulation, since somescaling may be required before summing the values.

[0110] Alternatively, as shown in FIG. 21, the received digital signal142 may be demodulated in parallel using both full-rate and enhancedhalf-rate branches. The transmitted digital signal 142 is received by aswitch 188 at the receiver 124, which initially transmits the receivedsignal to both the full-rate demodulation 190 and half-rate demodulation192 branches. The signal is demodulated and processed by the full-ratedemodulation branch 190 as if it were transmitted with full-rateπ/4-DQPSK modulation, while at the same time the signal is demodulatedand processed by the half-rate modulation branch 192 as if it weretransmitted with half-rate π/4-DBPSK modulation. Both branches result indemodulated, decoded data, and the result of the CRC decoder 150 in eachbranch is fed to the analyzer 159. The analyzer 164 determines in whichbranch the CRC code checks out, and sends a signal to the switch 188instructing the switch 188 to activate only that particular branch.

[0111] A second embodiment of the high performance enhanced half-rateencoder applies generally to radiotelephone systems and is notrestricted to D-AMPS or differential phase-shift modulation. This secondembodiment uses additional channel encoding to expand the half-ratesignal to fit the full-rate frame structure.

[0112] The bits output by a source encoder have different importance tothe fidelity of the reconstructed signal. Errors in some bits cause morepsychoacoustic disruption to the reconstructed audio signal than errorsin other bits. To optimize the fidelity of the reconstructed signal,bits output by the source encoder are generally assigned to classesranked in importance and, based on the class, the channel encoderprovides different degrees of redundancy to each class.

[0113] For example, as shown in FIG. 22, at the prior art transmitter193, the 112 bits produced every 20 milliseconds by a half-rate encoder194 are classified as twenty two critical bits, seventy three importantbits, and seventeen unimportant bits. The critical bits are encoded by aCRC encoder 196 which produces three parity bits. Further, the twentytwo critical bits are combined with the seventy three unimportant bits,and with six tail bits, by combiner 198. The one hundred and one bitsoutput from combiner 198 are encoded by a ½ rate convolutional encoder200, producing two hundred and two bits therefrom. The three parity bitsare encoded by a ⅓ rate convolutional encoder 202, which produces ninebits. Thus, there is a grand total two hundred and eleven encoded bits.The seventeen unimportant bits are not encoded but are combined with thetwo hundred and eleven protected bits bringing the total bit count totwo hundred and twenty eight bits per 20 milliseconds.

[0114] In the second embodiment in transmitter 203, the half-rateencoder of the transmitter of FIG. 22 is enhanced, as shown in FIG. 23.The half-rate encoder 194 again produces twenty two critical bits,seventy three important bits, and seventeen unimportant bits. The twentytwo critical bits are encoded by the CRC encoder 196 which producesthree parity bits. The twenty two critical bits are combined with theseventy three important bits, and with six tail bits, by the combiner198, producing one hundred and one bits. The one hundred and one bitsoutput by the combiner 198 are encoded by a ¼ rate convolutional encoder204, producing four hundred and four bits therefrom. The three paritybits are encoded by a ⅙ rate convolutional encoder 206, producingeighteen bits and giving a grand total of 422 encoded bits. Theseventeen unimportant bits are encoded by a ½ rate convolutional encoder208, producing thirty four bits therefrom, for a grand total of fourhundred and fifty six bits, or twice the ordinary output of thehalf-rate encoder every 20 milliseconds. This arrangement not onlyprovides greater overall protection against transmission errors, but italso provides some degree of protection to all bits.

[0115] A prior art variant of the encoder of FIG. 22 for use with aD-AMPS system is shown in FIG. 24. It is assumed that the half-rateencoder is being employed on a full-rate channel. Generally, a full-rateIS-136 channel has a capacity of 260 bits every 20 milliseconds, whereasa half-rate channel requires a capacity of 260 bits every 40milliseconds. A transmitter 209 is shown for transmitting on a full-ratechannel. The transmitter 209 includes a full-rate encoder 210 producingthree classes of bits, namely, class 1A (critical), class 1B (important)and class 2 (unimportant). The class 1A and 1B bits are encoded by a ½rate convolutional encoder 212. The output of the encoder 212 and theclass 2 bits are combined to produce 260 bits every 20 milliseconds.

[0116] Referring to FIG. 25, a transmitter 215 is shown for transmittinga half-rate encoded signal on a full-rate channel. The transmitter 215includes a half-rate encoder 214 producing three classes of bits,namely, class 1A, class 1B and class 2. However, in order to increasethe output of the half-rate encoder 214 to utilize the full-rate channelcondition, additional encoding is necessary. The class 1A and 1B bitsare encoded by a ¼ rate convolutionar encoder 216. The class 2 bits areencoded by a ½ rate convolutional encoder 218. The output of theencoders 216 and 218 are combined and produce 260 bits every 20milliseconds, thus filling the full-rate channel conditions.

[0117] While the additional encoding illustrated in FIG. 25 minimizesthe FER (Frame Error Rate) of the half-rate encoder 214, there is acrossover point in the performance of the half-rate encoder 214 incomparison to the full-rate encoder 210, as shown in FIG. 26.

[0118] It should be understood that the additional encoding of FIG. 25is exemplary. Other supplemental encoding could be used. Moreover, otheradditional encoding could be utilized, such as, but not limited to,trellis-coded modulation, block-coded modulation, product codes, and thelike.

[0119] In a third embodiment of enhanced half-rate channel encoding,each encoded 20 millisecond speech segment is twice repeated. As shownin FIG. 27, a prior art full-rate encoder 220 produces five hundred andtwenty bits every 40 milliseconds resulting in two hundred and sixtyQPSK symbols. As shown in FIG. 28, each 20 millisecond speech segmentoutput of a half-rate encoder 222 is repeated, resulting in two hundredand sixty QPSK symbols, the same as if the full-rate encoder 220 wasutilized.

[0120] Repetition of the 20 millisecond speech segments, results in anapproximate 3 dB gain in performance as shown in the curves of enhancedhalf-rate and full-rate encoding of FIG. 29. Further, the FER isminimized at low SNR. Moreover, there is potential for diversity gain,since channel fading can be different on the two repeats. However, underlow SNR conditions, this diversity gain will be marginal. It should beunderstood that if a GSM system is implemented, two hundred and twentyeight bits would be sent in the first slot of the full-rate pair, andthe same two hundred and twenty eight bits would be repeated in thesecond slot of the pair.

[0121] As shown in FIG. 30, a radiotelephone 224 includes both atransmitter 226 and a receiver 228. The receiver 228 measures thequality of received signals 229, and from this measurement determineswhether its transmitter 226 should use conventional full-rate orhalf-rate encoding or expanded half-rate encoding, via signal 230. Thisassumes that the RF channel provides symmetric performance. When thereceiver 228 senses that extensive transmission errors are occurring,the transmitter 226 is configured to use expanded half-rate encoding.The expanded half-rate encoding may be accomplished via any of themethods previously described.

[0122] As an example, if extensive errors are occurring, a decision maybe made by the receiver 228 to shift from conventional encoding (eitherfull-rate or half-rate) to expanded half-rate encoding. The receiver228, in addition to instructing the transmitter 226 to transmit inexpanded half-rate mode, via signal 230, also shifts from conventionaldecoding (either full-rate or half-rate) to expanded half-rate decoding.Once in expanded half-rate mode, a decision may be made by the receiver228 to shift back to conventional encoding (either full-rate orhalf-rate) if the error rate drops. In this manner, the near-end memberof a near-end-to-far-end conversation makes its own determinationconcerning the toggle of the near-end member from one mode to the otherbased on the assumption of the symmetric performance.

[0123] Alternatively, as shown in FIG. 31, the far-end member 232(radiotelephone) of a conversation pair can detect deteriorating channelconditions on signal 233, and request or instruct the near-end member234 (radiotelephone) to toggle, via signal 236, thereby mooting anyquestions of symmetry.

[0124] In any case, the second radiotelephone 232 at the far end of aconversation pair must somehow be aware that the near-end radiotelephone234 has toggled. The far-end radiotelephone 232 may be implicitlynotified of the toggle. For example, the far-end radiotelephone 232 maydecode the incoming data as if conventional full-rate transmission hasbeen utilized, and if errors are detected, toggle to enhanced half-ratemode. As an alternative, the far-end radiotelephone 232 could decode allincoming data according to both expanded half-rate and conventionalfull-rate decoding and select the most likely result of the two outputstreams, thereby deducing the mode of the near-end radiotelephone 234.As another alternative, the near-end radiotelephone 234 can explicitlyinform the far-end radiotelephone 232 that the near-end radiotelephone234 has toggled over a signalling channel 238, such as, but not limitedto, the SACCH or FACCH (Fast Associated Control Channel) of a cellularsystem.

[0125] Notification to toggle can also be carried from the far endradiotelephone 232 to the near end radiotelephone 234 by retransmissionrequests in an ARQ (Automatic Request for Retransmission) system.Alternatively, the far-end radiotelephone 232 can request or command thenear-end radiotelephone 234 to toggle over a signalling channel such asthe SACCH or FACCH of a cellular system.

[0126] While the present invention has been described with particularreference to the drawings, it should be understood that variousmodifications could be made without departing from the spirit and scopeof the present invention.

1. A method of transmitting a digitized signal across a radio channelcomprising the steps of: encoding a digitized signal at a first rate;expanding the encoded digitized signal to a second rate greater than thefirst rate; and transmitting the expanded digitized signal at the secondrate across a radio channel.
 2. The method of claim 1 , wherein theencoded digitized signal comprises a binary bit stream, and wherein thestep of expanding the encoded digitized signal to a second rate greaterthan the first rate comprises the step of repeating each bit in thebinary bit stream.
 3. The method of claim 1 , wherein the encodeddigitized signal comprises a binary bit stream, and wherein the step ofexpanding the encoded digitized signal to a second rate greater than thefirst rate comprises the step of repeating the binary bit stream.
 4. Themethod of claim 1 , wherein the encoded digitized signal comprises abinary bit stream having bits classified as one of critical, importantand unimportant, and wherein the step of expanding the encoded digitizedsignal to a second rate greater than the first rate comprises the stepof additionally encoding the binary bit stream according to bitclassification.
 5. The method of claim 4 , wherein the step ofadditionally encoding the binary bit stream according to bitclassification comprises the steps of: deriving parity bits from thecritical bits; encoding the parity bits to produce a first outputsignal; combining the critical and important bits and adding six tailbits to produce a second output signal; encoding the second outputsignal to produce a third output signal; encoding the unimportant bitsto produce a fourth output signal; and combining the first, third andfourth output signals to produce the expanded digitized signal at thesecond rate.
 6. The method of claim 5 , wherein the step of encoding theparity bits to produce a first output signal comprises the step ofencoding the parity bits with a ⅙ rate convolutional encoder; the stepof encoding the second output signal to produce a third output signalcomprises the step of encoding the second output signal with a ¼ rateconvolutional encoder; and the step of encoding the unimportant bits toproduce a fourth output signal comprises the step of encoding theunimportant bits with a ½ rate convolutional encoder.
 7. The method ofclaim 1 , wherein the digitized signal comprises a TDM (Time DivisionMultiplex) signal.
 8. The method of claim 1 , wherein the second rate isapproximately 2× the first rate.
 9. A method of establishing voicecommunication across a radio channel in a wireless communication system,said method comprising the steps of: transmitting a digitized speechsignal at a full-rate across a radio channel, said digitized speechsignal selected from the group consisting of (a) a full-rate encodeddigitized speech signal comprising a stream of binary bits, and (b) ahalf-rate encoded digitized speech signal comprising a stream of binarybits expanded for transmission at the full-rate by repeating each bit inthe binary bit stream; receiving the transmitted digitized speech signalat a receiver; determining whether the received digitized speech signalis the full-rate or half-rate encoded digitized speech signal; andactivating either a full-rate equalizer or a half-rate equalizer at thereceiver in response to said determination to demodulate the receiveddigitized speech signal.
 10. The method of claim 9 , wherein the stepsof determining whether the received digitized speech signal is thefull-rate or half-rate encoded digitized speech signal and activatingeither a full-rate equalizer or a half-rate equalizer at the receiver inresponse to said determination to demodulate the received digitizedspeech signal comprise the steps of: demodulating the received digitizedspeech signal at the full-rate equalizer, the full-rate equalizerproducing dibits of information in response thereto; inputting thedibits of information to an XNOR gate, said XNOR gate outputting alogical one if the bits of the dibit are the same and a logical zero ifthe bits of the dibit are different; counting the number of occurrencesof logical ones at the output of the XNOR gate; and activating thehalf-rate equalizer to demodulate the received digitized speech signalif the number of logical ones exceeds a threshold value.
 11. The methodof claim 9 , wherein the steps of determining whether the receiveddigitized speech signal is the full-rate or half-rate encoded digitizedspeech signal and activating either a full-rate equalizer or a half-rateequalizer at the receiver in response to said determination todemodulate the received digitized speech signal comprise the step of:demodulating the received digitized speech signal at the full-rateequalizer, the full-rate equalizer producing dibits of soft informationin response thereto, each soft dibit including soft values; multiplyingthe soft values of the soft dibit together at a multiplier, saidmultiplier outputting a positive value if the soft values of the softdibit are the same and a negative value if the soft values of the softdibit are different; summing the output of the multiplier; andactivating the half-rate equalizer to demodulate the received digitizedspeech signal if the summed output of the multiplier exceeds a thresholdvalue.
 12. The method of claim 9 , wherein the steps of determiningwhether the received digitized speech signal is the full-rate orhalf-rate encoded digitized speech signal and activating either afull-rate equalizer or a half-rate equalizer at the receiver in responseto said determination to demodulate the received digitized speech signalcomprise the steps of: demodulating the received digitized speech signalat the full-rate equalizer, the full-rate equalizer producing dibits ofsoft information in response thereto, said soft dibits plottable on adifferential constellation having real and imaginary axes, each plottedsoft dibit representing a complex value of a differential symbol;rotating the differential symbols by π/4, said rotated differentialsymbols having components on the real and imaginary axes; summingmagnitudes of the rotated differential symbol components on the real andimaginary axes; calculating a ratio of real axis summed magnitudesversus imaginary axis summed magnitudes; and activating the half-rateequalizer to demodulate the received digitized speech signal if theratio exceeds a threshold value.
 13. The method of claim 9 , wherein thesteps of determining whether the received digitized speech signal is thefull-rate or half-rate encoded digitized speech signal and activatingeither a full-rate equalizer or a half-rate equalizer at the receiver inresponse to said determination to demodulate the received digitizedspeech signal comprise the steps of: demodulating the received digitizedspeech signal at the full-rate equalizer, the full-rate equalizerproducing dibits of soft information in response thereto, each softdibit representing a differential symbol; determining phase differencesbetween successive differential symbols; summing the determined phasedifferences; and activating the half-rate equalizer to demodulate thereceived digitized speech signal if the summed phase difference exceedsa threshold value.
 14. The method of claim 9 , wherein the steps ofdetermining whether the received digitized speech signal is thefull-rate or half-rate encoded digitized speech signal and activatingeither a full-rate equalizer or a half-rate equalizer at the receiver inresponse to said determination to demodulate the received digitizedspeech signal comprise the steps of: demodulating the received digitizedspeech signal in parallel using both full-rate and half-ratedemodulation branches; performing CRC (Cyclic Redundancy Check) checkson the demodulated full-rate and half-rate signals; and deactivating oneof the full-rate and half-rate demodulation branches in response to theCRC checks.
 15. The method of claim 9 , wherein the digitized speechsignal comprises a TDM (Time Division Multiplex) signal.
 16. The methodof claim 9 , wherein the steps of determining whether the receiveddigitized speech signal is the full-rate or half-rate encoded digitizedspeech signal and activating either a full-rate equalizer or a half-rateequalizer at the receiver in response to said determination todemodulate the received digitized speech signal comprise the steps of:demodulating the received digitized speech signal at the full-rateequalizer, the full-rate equalizer producing dibits of soft informationin response thereto, said soft dibits plottable on a differentialconstellation having real and imaginary axes, each plotted soft dibitrepresenting a complex value of a differential symbol; rotating thedifferential symbols by π/4, said rotated differential symbols havingcomponents on the real and imaginary axes; squaring the complex valuesof the rotated differential symbols; summing the squared values; andactivating the half-rate equalizer to demodulate the received digitizedspeech signal if the real component of the squared/summed value exceedsa threshold value.
 17. A method of receiving a digitized speech signaltransmitted at a full-rate across a radio channel in a wirelesscommunication system, the digitized speech signal selected from thegroup consisting of (a) a full-rate encoded digitized speech signalcomprising a stream of binary bits, and (b) a half-rate encodeddigitized speech signal comprising a stream of binary bits expanded fortransmission at the full-rate by repeating each bit in the binary bitstream, said method comprising the steps of: receiving the digitizedspeech signal at a receiver; determining whether the received digitizedspeech signal is the full-rate or half-rate encoded digitized speechsignal; and activating either a full-rate or a half-rate equalizer atthe receiver in response to said determination to demodulate thereceived digitized speech signal.
 18. The method of claim 17 , whereinthe steps of determining whether the received digitized speech signal isthe full-rate or half-rate encoded digitized speech signal andactivating either a full-rate equalizer or a half-rate equalizer at thereceiver in response to said determination to demodulate the receiveddigitized speech signal comprise the steps of: demodulating the receiveddigitized speech signal at the full-rate equalizer, the full-rateequalizer producing dibits of information in response thereto; inputtingthe dibits of information to an XNOR gate, said XNOR gate outputting alogical one if the bits of the dibit are the same and a logical zero ifthe bits of the dibit are different; counting the number of occurrencesof logical ones at the output of the XNOR gate; and activating thehalf-rate equalizer to demodulate the received digitized speech signalif the number of logical ones exceeds a threshold value.
 19. The methodof claim 17 , wherein the steps of determining whether the receiveddigitized speech signal is the full-rate or half-rate encoded digitizedspeech signal and activating either a full-rate equalizer or a half-rateequalizer at the receiver in response to said determination todemodulate the received digitized speech signal comprise the steps of:demodulating the received digitized speech signal at the full-rateequalizer, the full-rate equalizer producing dibits of soft informationin response thereto, each soft dibit including soft values; multiplyingthe soft values of the soft dibit together at a multiplier, saidmultiplier outputting a positive value if the soft values of the softdibit are the same and a negative value if the soft values of the softdibit are different; summing the output of the multiplier; andactivating the half-rate equalizer to demodulate the received digitizedspeech signal if the summed output of the multiplier exceeds a thresholdvalue.
 20. The method of claim 17 , wherein the steps of determiningwhether the received digitized speech signal is the full-rate orhalf-rate encoded digitized speech signal and activating either afull-rate equalizer or a half-rate equalizer at the receiver in responseto said determination to demodulate the received digitized speech signalcomprise the steps of: demodulating the received digitized speech signalat the full-rate equalizer, the full-rate equalizer producing dibits ofsoft information in response thereto, said soft dibits plottable on adifferential constellation having real and imaginary axes, each plottedsoft dibit representing a complex value of a differential symbol;rotating the differential symbols by π/4, said rotated differentialsymbols having components on the real and imaginary axes; summingmagnitudes of the rotated differential symbol components on the real andimaginary axes; calculating a ratio of real axis summed magnitudesversus imaginary axis summed magnitudes; and activating the half-rateequalizer to demodulate the received digitized speech signal if theratio exceeds a threshold value.
 21. The method of claim 17 , whereinthe steps of determining whether the received digitized speech signal isthe full-rate or half-rate encoded digitized speech signal andactivating either a full-rate equalizer or a half-rate equalizer at thereceiver in response to said determination to demodulate the receiveddigitized speech signal comprise the steps of: demodulating the receiveddigitized speech signal at the full-rate equalizer, the full-rateequalizer producing dibits of soft information in response thereto, eachsoft dibit representing a differential symbol; determining phasedifferences between successive differential symbols; summing thedetermined phase differences; and activating the half-rate equalizer todemodulate the received digitized speech signal if the summed phasedifference exceeds a threshold value.
 22. The method of claim 17 ,wherein the steps of determining whether the received digitized speechsignal is the full-rate or half-rate encoded digitized speech signal andactivating either a full-rate equalizer or a half-rate equalizer at thereceiver in response to said determination to demodulate the receiveddigitized speech signal comprise the steps of: demodulating the receiveddigitized speech signal in parallel using both full-rate and half-ratedemodulation branches; performing CRC (Cyclic Redundancy Check) checkson the demodulated full-rate and half-rate signals; and deactivating oneof the full-rate and half-rate demodulation branches in response to theCRC checks.
 23. The method of claim 17 , wherein the digitized speechsignal comprises a TDM (Time Division Multiplex) signal.
 24. The methodof claim 17 , wherein the digitized speech signal comprises a TDM (TimeDivision Multiplex) signal.
 25. In a transmitter for transmittingcommunication signals across a radio channel, an improved encodercomprising: a half-rate encoder receiving a digitized speech signal andgenerating a compressed bit stream at half-rate; and a signal expanderreceiving the compressed bit stream and generating an expanded bitstream at full-rate for transmission across a radio channel.
 26. Theimproved encoder of claim 25 , wherein the full-rate is approximately 2×the half-rate.
 27. The improved encoder of claim 25 , wherein the signalexpander comprises a repeater repeating each bit in the compressed bitstream to generate the expanded bit stream.
 28. The improved encoder ofclaim 25 , wherein the signal expander comprises a repeater repeatingthe compressed bit stream to generate the expanded bit stream.
 29. Theimproved encoder of claim 25 , wherein the compressed bit streamincludes bits classified as one of critical, important and unimportant,and wherein the signal expander comprises a plurality of encodersadditionally encoding the compressed according to bit classification togenerate the expanded bit stream.
 30. An improved receiver for receivinga digitized speech signal transmitted at a full-rate across a radiochannel in a wireless communication system, the digitized speech signalselected from the group consisting of (a) a full-rate encoded digitizedspeech signal comprising a stream of binary bits, and (b) a half-rateencoded digitized speech signal comprising a stream of binary bitsexpanded for transmission at the full-rate by repeating each bit in thebinary bit stream, the improved receiver comprising: a full-rateequalizer; a half-rate equalizer; a switch initially routing thereceived digitized speech signal to the full-rate equalizer, whereinsaid full-rate equalizer demodulates the received digitized speechsignal producing a full-rate demodulated signal and dibits ofinformation corresponding to the full-rate demodulated signal; and ananalyzer analyzing the dibits of information, said analyzer controllingthe switch to route the received digitized speech signal to one of thefull-rate and half-rate equalizers based upon said analysis.
 31. Theimproved receiver of claim 30 , wherein said analyzer comprises: an XNORgate receiving the dibits of information, said XNOR gate outputting alogical one if the bits of the dibit are the same and a logical zero ifthe bits of the dibit are different; a counter receiving the output ofthe XNOR gate, said counter counting the occurrences of logical ones atthe output of the XNOR gate; and a threshold detector connected to thecounter, said threshold detector controlling the switch to route thereceived digitized speech signal to the half-rate equalizer if thenumber of logical ones counted by the counter exceeds a threshold value.32. The improved receiver of claim 30 , wherein said dibits ofinformation comprise dibits of soft information, each soft dibitincluding soft values, and wherein said analyzer comprises: a multipliermultiplying the soft values of the soft dibits together, said multiplieroutputting a positive value if the soft values of the soft dibit are oflike polarity and negative value if the soft values of the soft dibitare of different polarity; a summer receiving and summing the output ofthe multiplier; and a threshold detector connected to the summer, saidthreshold detector controlling the switch to route the receiveddigitized speech signal to the half-rate equalizer if the summed valueexceeds a positive threshold value.
 33. The improved receiver of claim30 , wherein said dibits of information comprise dibits of softinformation plottable on a differential constellation having real andimaginary axes, each plotted dibit representing a complex value of adifferential symbol, and wherein said analyzer comprises: a rotatorrotating the differential symbols by π/4, said rotated differentialsymbols having components on the real and imaginary axes; a summersumming magnitudes of the rotated differential symbol components on thereal and imaginary axes and calculating a ratio of real axis summedmagnitudes versus imaginary axis summed magnitudes; and a thresholddetector connected to the summer, said threshold detector controllingthe switch to route the received digitized speech signal to thehalf-rate equalizer if the calculated ratio exceeds a threshold value.34. The improved receiver of claim 30 , wherein said dibits ofinformation comprise dibits of soft information plottable on adifferential constellation having real and imaginary axes, each plotteddibit representing a complex value of a differential symbol, and whereinsaid analyzer comprises: a rotator rotating the differential symbols byπ/4, said rotated differential symbols having components on the real andimaginary axes; a squarer and summer squaring the complex values of therotated differential symbols and summing the squared values; and athreshold detector connected to the squarer and summer, the thresholddetector controlling the switch to route the received digitized speechsignal to the half-rate equalizer if the real component of thesquared/summed value exceeds a threshold value.
 35. The improvedreceiver of claim 30 , wherein said dibits of information comprisedibits of soft information having a real number magnitude and a realnumber phase plottable on a differential constellation, each plotteddibit representing a differential symbol, and wherein said analyzercomprises: a phase differentiator determining phase changes betweensuccessive differential symbols; a summer summing the phase changesdetermined by the phase differentiator; and a threshold detectorconnected to the summer, said threshold detector controlling the switchto route the received digitized speech signal to the half-rate equalizerif the summed phase changes exceed a threshold value.
 36. An improvedreceiver for receiving a digitized speech signal transmitted at afull-rate across a radio channel in a wireless communication system, thedigitized speech signal selected from the group consisting of (a) afull-rate encoded digitized speech signal comprising a stream of binarybits, and (b) a half-rate encoded digitized speech signal comprising astream of binary bits expanded for transmission at the full-rate, theimproved receiver comprising: a full-rate demodulation branch comprisinga full-rate equalizer and a first CRC (Cyclic Redundancy Check) decoder;a half-rate demodulation branch comprising a half-rate equalizer and asecond CRC decoder; a switch receiving the digitized speech signal andinitially routing the received digitized speech signal to both thefull-rate and half-rate demodulation branches, wherein the receiveddigitized speech signal is received by the full-rate and half-rateequalizers producing full-rate demodulated and half-rate demodulatedsignals, respectively, said full-rate demodulated signal input to thefirst CRC decoder performing a CRC check on the full-rate demodulatedsignal and producing a first CRC check signal, and said half-ratedemodulated signal input to the second CRC decoder performing a CRCcheck on the half-rate demodulated signal and producing a second CRCcheck signal; and an analyzer analyzing the first and second CRC checksignals, said analyzer controlling the switch to route the receiveddigitized speech signal to one of the first and second demodulationbranches based on said analyzation.